Normal development, growth, and homeostasis in multicellular organisms require a careful balance between the production and destruction of cells in tissues throughout the body. Cell division is a carefully coordinated process with numerous checkpoints and control mechanisms. These mechanisms are designed to regulate DNA replication and to prevent inappropriate or excessive cell proliferation. In contrast, apoptosis is the genetically controlled process by which unneeded or damaged cells can be eliminated without causing the tissue destruction and inflammatory responses that are often associated with acute injury and necrosis.
The term "apoptosis" was first used by Kerr, J. F. et al. (1972; Br. J. Cancer 26:239-257) to describe the morphological changes that characterize cells undergoing programmed cell death. Apoptotic cells have a shrunken appearance with an altered membrane lipid content and highly condensed nuclei. Apoptotic cells are rapidly phagocytosed by neighboring cells or macrophages without leaking their potentially damaging contents into the surrounding tissue or triggering an inflammatory response.
The processes and mechanisms regulating apoptosis are highly conserved throughout the phylogenetic tree, and much of the current knowledge about apoptosis is derived from studies of the nematode, Caenorhabditis elegans and the fruit fly, Drosophila melanogaster (cf., Steller, H. (1995) Science 267:1445-1449, and references therein). Dysregulation of apoptosis has recently been recognized as a significant factor in the pathogenesis of human disease. For example, inappropriate cell survival can cause or contribute to many diseases such as cancer, autoimmune diseases, and inflammatory diseases. In contrast, increased apoptosis can cause immunodeficiency diseases such as AIDS, neurodegenerative disorders, and myelodysplastic syndromes (Thompson, C. B. (1995) Science 267:1456-1462).
A variety of ligands, enzymes, tumor suppressors, viral gene products, pharmacological agents, inorganic ions, and their cellular receptors have important positive or negative roles in regulating and implementing the apoptotic destruction of a cell. Although some specific components of the apoptotic pathway have been identified and characterized, many interactions between the proteins involved are undefined, leaving major aspects of the pathway unknown (Steller, H., supra; Thompson, C. B., supra).
The p53 protein has been shown to function as a tumor suppressor by initiating apoptosis in cells undergoing deregulated growth. p53 has been proposed to mediate its effects by stimulating or repressing the transcription of particular sets of target genes in a gene and tissue-specific manner. For example, certain cellular genes are transcriptionally activated by p53 through p53-responsive elements in their promoter regions. In contrast, several genes that lack p53-responsive elements are repressed by p53. Additionally, the fact that p53 has also been shown to regulate activity of certain genes either positively or negatively depending on specific cell type suggests the involvement of cell-specific factors or coregulators in p53-mediated transcriptional regulation. (Debbas, M. and White, E. (1993) Genes Dev 7:546-554; Zhao, J. et al. (1994) Mol Cell Biol 14:8483-8492; Chin, K. et al. (1992) Science 255:459-462; Jackson, P. et al. (1994) Biochem Biophys Res Commun 203:133-140).
The cytotoxic effect of many chemotherapeutic drugs is dependent on their ability to trigger apoptosis through a p53-dependent pathway. Only two p53-regulated genes, bcl-2 and bax, have demonstrated functions in the apoptotic pathway. Another gene, E124, which is expressed during p53-mediated apoptosis in mouse cells, may be an additional candidate for a gene involved in implementing this pathway. E124 was isolated by using a differential display technique designed to identify RNA species whose expression is altered during apoptosis. Expression of functional p53 in cells transformed with oncogenes (E1A and T24-H-ras) was found to be sufficient to induce E124 mRNA. In addition, E124 mRNA is rapidly induced in cells undergoing p53-mediated apoptosis, such as irradiated mouse thymocytes and NIH3T3 cells treated with etoposide. In untreated cells and treated cells that lack p53 function E124 is only present at low levels. The sequence of E124 seems to be evolutionarily conserved for it demonstrates 36% identity in the most related regions, and 25% overall homology, with C. elegans CELF37 amino acid sequence (Clarke, A. et al. (1993) Nature 362:849-852; Mishiyata, T., et al (1994) Oncogene 9:1799-1805; Lehar, S. M. et al. (1996) Oncogene 12:1181-1187).
The discovery of proteins related to mouse E124 protein, and the polynucleotides encoding them, satisfies a need in the art by providing new compositions useful in diagnosis and treatment of disorders associated with increased or decreased apoptosis.